Heat transfer surface structure

ABSTRACT

A heat transfer surface structure is described wherein both heat and vaporizable liquid are conveyed through a capillary material to a free vaporizing surface of the capillary material where the liquid vaporizes. Heat is conducted from a heat source wall through a portion of the capillary material to the vaporizing surface where it escapes as heat of vaporization along with the vapor. The liquid flows through the pores of the capillary material from a liquid source to the vaporizing surface under the influence of capillary forces. The vaporizing surface is divided into a large number of regions which are close to the heat source wall and are connected by way of vapor passages to a region external to the capillary material. Thus, the heat conducting paths through the capillary material are very short, and vapor can escape freely through relatively large passages rather than having to force its way through the pores of the capillary material where it would interfere with the liquid flow. Such a vented capillary vaporizer is capable of handing much higher heat flux densities than previous capillary vaporizers. Four examples of capillary vaporizer are set forth, two of these for operation where the liquid and vapor are comingled as in a boiler tube or evaporator tube. One of these has separated areas of capillary material in thermal contact with the heat source surface, thereby defining passages therebetween. The other is similar with added portions of porous materials to form a manifold having a hierarchy of vapor passages of decreasing number and increasing cross section, thus increasing the separation between the regions of liquid input and vapor output. The third example accepts liquid from a capillary structure or wick as in a heat pipe, and also has a pair of manifolds in the form of a hierarchy of passages for vapor flow and capillary paths for liquid flow. The fourth example receives bulk liquid through a channel, and delivers the vapor through a separate passage. Thermal insulation maintains the bulk liquid relatively cool, and active cooling may be provided. This latter embodiment is unique in its ability to pump the heat transfer fluid since the output vapor can be at a higher pressure than the incoming bulk liquid.

United States Patent [72] Inventor Robert David Moore, Jr.

817 W. Cmho RM, Arcadia, Calif. 91006 (21] Appl. No. 52,609 [22] FiledJuly 6, 1970 [45] Patented Aug. 10, 1971 [54] HEAT TRANSFER SURFACESTRUCTURE 28C1a1m,21l)nwing1 fls.

521 user. 165/133, 165/105, 165/180, 165/181 [51] lntJ F28f13/06 [50]FleldofSeareh 165/105 OTHER REFERENCES Stenger; F. 1., ExperimentalFeasibility Study Of Water- Filled Capillary-Pumped Heat-TransferLoops," NASA Tech. Memorandum (TMX- 1310), 11/1966 PrimaryExaminerAlbert W. Davis, Jr. Attorney-Christie, Parker and HaleABSTRACT: A heat transfer surface structure is described wherein bothheat and vaporizable liquid are conveyed through a capillary material toa free vaporizing surface of the capillary material where the liquidvaporizes. Heat is conducted from a heat source wall through a portionof the capillary material to the vaporizing surface where it escapes asheat of vaporization along with the vapor. The liquid flows through thepores of the capillary material from a liquid source to the vaporizingsurface under the influence of capillary forces. The vaporizing surfaceis divided into a large number of regions which are close to the heatsource wall and are connected by way of vapor passages to a regionexternal to the capillary material. Thus, the heat conducting pathsthrough the capillary material are very short, and vapor can escapefreely through relatively large passages rather than having to force itsway through the pores of the capillary material where it would interferewith the liquid flow. Such a vented capillary vaporizer is capable ofhanding much higher heat flux densities than previous capillaryvaporizers. Four examples of capillary vaporizer are set forth, two ofthese for operation where the liquid and vapor are comingled as in aboiler tube or evaporator tube. One of these has separated areas ofcapillary material in thermal contact with the heat source surface,thereby defining passages therebetween. The other is similar with addedportions of porous materials to form a manifold having a hierarchy ofvapor passages of decreasing number and increasing cross section, thusincreasing the separation between the regions of liquid input and vaporoutput. The third example accepts liquid from a capillary structure orwick as in a heat pipe, and also has a pair of manifolds in the form ofa hierarchy of passages for vapor flow and capillary paths for liquidflow. The fourth example receives bulk liquid through a channel, anddelivers the vapor through a separate passage. Thermal insulationmaintains the bulk liquid relatively cool, and active cooling may beprovided. This latter embodiment is unique in its ability to pump theheat transfer fluid since the output vapor can be at a higher pressurethan the incoming bulk liquid.

PATENTEDAUBIOIQYI 3,598,180

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PATENTED M910 1971 SHEET 3 OF 4 \\V m% I a u.

BACKGROUND Filed simultaneously herewith are two closely related patentapplications by Robert David Moore, .lr., entitled Segmented Heat Pipe",Ser. No. 52,249 and The Heat Link, A Heat Transfer Device With IsolatedFluid Flow Paths" Ser. No. $2,642, each of which describes and claimsheat transfer apparatus, including heat transfer surface structuresincorporating principles of this invention. The content of thesecopending patent applications is hereby incorporated by reference forfull force and effect as if set forth in full herein.

vaporization heat transfer is widely employed in a broad variety ofboilers, heat exchangers, heat pipes and the like. In the ordinaryboiler, where bulk liquid and vapor are comingled for contact with theheat transfer surface, vaporization 'takes place in three well-knownstages as the energy input through the solid surface into the liquid isincreased. Initially, the liquid is warmed and vaporization takes placefrom the liquid surface without the formation of bubbles. In the secondstage, so-called nucleate boiling occurs wherein vapor bubbles form atthe heated surface and pass through the bulk liquid to give a very highheat transfer rate. As the heat flux through the surface into the liquidrises, the vapor bubbles form at a higher rate and at closer spacing sothat eventually they form a substantially continuous film of vapor overthe surface. The thermal transfer characteristics of the vapor areappreciably lower than that of the liquid, and this so-called filmboiling results in a sharp decrease in heat transfer away from the heatinput surface. Thus causes a sharp increase in temperature of thesurface, and not only is the heat transfer rate decreased, but alsothere is a danger of damage to the surface due to excessivetemperatures. In order to maximize heat flux, it is desirable to preventthe formation of a continuous vapor film at the heat transfer surface.

In a conductively heated capillary vaporizer as in a heat pipe asomewhat different arrangement is employed than in a boiler, since in aboiler vaporization takes place at the heat input surface which isnormally just a sheet of metal or the like, and in a heat pipe a porouscapillary material is employed for-conveying both the liquid and theheat to the surface or region of vaporization. Although the heat fluxrates obtainable with a conductively heated capillary vaporizer arequite high, they are limited by the rate at which vapor can escape fromthe surface or region of vaporization or the tendency of the vapor todrive the liquid out of the hotter portions of the porous matrix, thuscutting off the supply of liquid to the surface or region at whichvaporization occurs. These limitations are serious since the volume ofvapor formed is quite high compared with the volume of liquid, and thevapor must either be formed at the surface of the porous capillarymaterial following heat conduction through the capillary material, inwhich case a considerable portion of the capillary material becomeshotter than the vapor, or the vapor must pass through the pores of thecapillary material with substantial flow resistance. It is, therefore,desirable to provide a heat transfer surface structure which willdeliver heat and liquid to a surface of vaporization and remove vaporfrom the surface with minimized thermal and fluid flow resistances.

BRIEF SUMMARY OF THE INVENTION Thus, in practice of this inventionaccording to a preferred embodiment, there is provided a heat transfersurface on a heatsource surface. A capillary matrix, wet by avaporizable liquid, has a first surface portion in thermal contact withthe heat source, a second surface portion in contact with thevaporizable liquid, and a third surface portion from which the principalvaporization of liquid from the capillary matrix takes place. The thirdsurface portion is arranged as a multiplicity of regional areassufficiently close to each other and to the heat source surface that thevolume of the capillary matrix, through which the liquid must passbetween the second surface portion and the third surface portion,remains liquid filled. A multiplicity of vapor passages sufficientlylarger than the capillary size of the capillary matrix to remain vaporfilled are in fluid communication between the regional areas and anotherregion external to the capillary matrix to which vapor is free to flow.

DRAWINGS The above mentioned and other features and advantages of thepresent invention will be better understood by reference to thefollowing detailed description of a presently preferred embodiment whenconsidered in connection with the accompanying drawings wherein:

FIG. A illustrates schematically a vapor-liquid interface at the end ofa capillary;

FIG. B illustrates a vapor bubble in a liquid-filled capillary;

FIG. 1 illustrates schematically a porous body for fluid and heattransfer;

FIG. 2 illustrates schematically a conductively heated capillary bodyfor fluid transfer and heat transfer;

FIG. 3 illustrates a simple heat transfer surface structureincorporating principles of this invention;

FIG. 4 illustrates in cross section a compound boiling surfaceincorporating principles of this invention;

FIG. 5 is another cross section of the structure of FIG. 4;

FIG. 6 illustrates schematically a fragment of heat transfer surfaceincorporating principles of this invention;

FIGS. 7, 8, 9 and 10 illustrate other embodiments of heat transfersurface structure related to that of FIG. 6;

FIG. 11 illustrates in transverse cross section a wick-fed vaporizerincorporating principles of this invention, such as may be used in aheat pipe;

FIG. 12 is a longitudinal cross section of the vaporizer of FIG. 11;

FIG. 13 is another transverse cross section of the vaporizer of FIG. 11;

FIG. 14 is another longitudinal cross section of the vaporizer of FIG. 11;

FIG. 15 is a perspective view of a fragment of the vaporizer of FIG. 11;

FIG. 16 is a magnified view of a portion of the vaporizer of FIG. 11;

FIG. 17 is a transverse cross section of a fragment of a capillary pumpvaporizer structure incorporating principles of this invention;

FIG. 18 is a cross section transverse to that of FIG. 17; and

FIG. 19 is a magnified view of a portion of the structure of FIG. 17.

Throughout the drawings, like reference numerals refer to like parts.

In order to obtain maximum heat flux rates from a vaporizer, it isnecessary to allow heat and the vaporizable liquid to reach theliquid-vapor interface at which vaporization of the liquid takes placeas easily as possible, and also allow the vapor formed to escape easily.In a boiler or the like where the surface is exposed to bulk liquid, theformation of a continuous vapor film inhibits the flow of vaporizableliquid to the heat transfer surface and the flow of heat to the liquid.Previous arrangements for conductively heated capillary Vaporizers havebeen limited in either the ability of the heat to reach the interface,or the liquid to reach the interface, or in the ability of vapor toleave the interface.

In a typical conventional heat pipe, the capillary vaporizer has arelatively thick layer of porous material adjacent the wall throughwhich heat enters the heat pipe in order to obtain an adequate liquidflow. Both heat and liquid must flow through this material. If the layerof porous material is too thin the resistance to liquid flow is high. Athigh heat flux the liquid may not reach across the entire porousvaporizer, and excessive temperatures may be encountered in dry" regionsof the vaporizer. If, on the other hand, the porous material is toothick, heat entering the heat pipe through the impermeable wall heatsthe porous material and the liquid therein in the region adjacent thewall. When the heat flux becomes high, the liquid adjacent the wall mayvaporize, but the vapor is effectively trapped by the liquid-filledporous material, and a situa tion somewhat analogous to film boiling isencountered. In this situation, heat must be conducted through both thevaporfilled porous material and the liquid-filled porous material toevaporate liquid from the free surface. Thus, in either a free boilingsituation or in a heat pipe, it is desirable to have means for conveyingliquid and heat to the surface where vaporization occurs with as littleresistance as possible, and also provide means for removing vapor fromthe vaporization region with minimum resistance.

In order to thoroughly appreciate the principles and operatingadvantages of the heat transfer surface structure provided in practiceof this invention and to define and develop the art sufliciently thatpractical use can be made of the invention, the operating principles ofconventional and improved capillary vaporizers are discussed herein.Emphasis is given to the limitations on the operation of a capillaryvaporizer, particularly the maximum heat flux rates obtainable, in orderto confirm the relative and absolute performance of the improvedcapillary vaporizer and to provide the theoretical basis needed foroptimal engineering design. in one of the improved vaporizers, pumping"can be obtained wherein the vapor leaving the capillary vaporizer is ata higher pressure than the liquid reaching the vaporizer.

The flow of liquid through a porous material under the influence ofsurface tension forces is analyzed first and the resulting equationapplied to demonstrate the performance of a radiantly heated vaporizer.The limiting heat flux of a simple conductively heated vaporizer is thenshown to be far lower, on the order of about 1 percent of that of theradiantly heated vaporizer, due to the small temperature differentialthat can be maintained across the porous material without vapordisplacing the liquid from the pores. A generalized improved vaporizeris then described that has numerous vapor passages close to the heatsource and close to each other so as to greatly reduce the distancebetween the heat source and those surfaces of the porous material fromwhich vapor can freely escape. Scaling laws and formulas for the maximumheat flux rate are presented which illustrate the extremely high heatflux capability of the improved vaporizer which, for example, is oftenover an order of magnitude better than previously available withconventional conductively heated capillary vaporizers or even themaximum available from nucleate boiling. Various geometries of theimproved vaporizer are then described and illustrated which allow thenew heat transfer surface to be used to advantage in such diverseapplications as boiler tubes, refrigerator evaporators, high heat fluxheat pipes, and advanced heat transfer systems such as described in theaforementioned copencling patent applications.

in the following discussion, an appreciable use of mathematicalequations is required for a full understanding of the subject matter tocalculate the performance of the improved vaporizer. In the followingdiscussion, symbols for the various quantities and parameters as setforth in the following table are employed. This table not only setsforth the symbols employed and their nature, but also typical units asemployed in examples herein.

heat conductivity of capillary matrix TABLE OF SYMBOLS-Continued heatconductivity of solid substance forming capillary matrix a fluidconductivity of capillary matrix with unit viscosity fluid 8 effectivematrix pore surface/volume ratio wick microstructurc efficiencyvaporizer microstructure efficiency G vaporizer geometrical factor Fpressure pressure drop in pores of capillary due to viscous drag bubblepressure of liquid filled capillary material pressure differentialavailable to drive liquid through porous material gravitational head atevaporator other pressure differential to overcome viscous drag offluids sum of pressure drops external to capillary matrix temperaturecontact angle liquid to vaporizer acceleration of gravity fluid flowrate heat flux height of surface of vaporization above bulk liquidsurface radius pore radius in capillary matrix area totalcross-sectional area of pores width of single vaporizer slab thicknessof single vaporizer slab length of heat pipe length or distance numberof vapor passages/unit length thickness of microslabs composingvaporizer matrix [7 thickness of microchannels in vaporizer matrix ddistance between vaporizing surface regions 5 length of vaporizerstrip-unit area of surface wutts/cm- K l/crn.

dyne/cm. AP,

dyne/cm. AP, dynelcm.

dyne/cm. AP,

dynelcm.

radians cmJsec. cm.'l sec.

watts l/cm.

Certain simplifying assumptions are also made throughout the followingdiscussion, and generally speaking the assumptions do not change theresults appreciably in practical situa tions, including the example setforth hereinafter. As with almost all simplifying mathematicalassumptions, extreme examples can be found where gross error wouldresult from the assumptions. The typical conventional capillaryvaporizers and the improved heat transfer structures hereinafterdescribed are not such examples.

It is assumed that liquid within the porous matrix wets the surface witha contact angle 0 equal to 0. This results only in a mathematicalsimplicity since if the contact angle is other than 0 the surfacetension 0 is merely replaced by a which is equal to 0' cos 0 in all ofthe formulas.

The heat conductivity k of the whole porous matrix is assumed to be thesame whether the pores are filled with liquid or vapor. This is a goodapproximation leading only to minor inaccuracies for water or organicfluids in a matrix of high thermal conductivity metal such as silver,aluminum, copper or gold. Where this approximation is poor, as in thecase where the fluid is a high-heat conductivity liquid metal, theparametric constants obtained when solving equations for limiting heatflux in a conductively heated vaporizer may change, but the mathematicalform of the equations does remain the same.

The porous material is assumed to have uniform pore size, fluidconductivity, bubble pressure, heat conductivity and the like throughoutits defined boundaries, though in some circumstances, as pointed outhereinafter, two or more porous materials with different parameters areemployed for improved performance. Practical, presently available porousmaterials rarely have uniform pore size, but calculations based onstatistically varying pore size distributions are unnecessarilycomplicated and in more circumstances would merely introduce a geometricfactor that would not alter the comparative merit of one structure ascompared with another. If it is desired to obtain a more exact solution,the fluid conductivity (1 may be measured empirically as a function ofthe difierence between vapor pressure and the fluid pressure in thematrix with the results employed directly in a numerical solution ratherthan calculating the fluid conductivity from pore size distributions.

Two idealized geometrical structures are utilized in the mathematicaltreatment, a wick matrix having a honeycomb like structure that isessentially of a bundle of uniform parallel tubes with very thin walls,and a vaporizer matrix comprising closely spaced parallel plates of highthermal conductivity which when optimized turn out to have a platethickness the same as the width of the gaps or channels between adjacentplates. While most presently available capillary matrix structures foreither wicks or Vaporizers do not approach these ideal structuresclosely, calculations based on the ideal structures determine therelationships between fluid conductivity a, eflective matrix poresurface to volume ratio 8, and the ratio of the heat conductivity of thecapillary matrix to the heat conductivity of the solid substance fonningthe capillary matrix (k /k and also identify ideal performance figureswith which other real matrix structures can be compared in order to ratetheir efficiency".

Analyzing first the flow through a porous material induced by surfacetension forces as applied to capillary Vaporizers, the flow rate perunit area F/A of a fluid of viscosity 1; through a porous materialhaving fluid conductivity at under a pressure gradient dp/dx is a tipAlso the maximum static pressure difierential or bubble pressure AP,that can be supported across a liquid vapor interface in a porousmaterial having an effective pore surface to pore volume ratio 8 andfilled with a liquid wetting the porous material with a zero contactangle, and having a surface tension 0 is APB 0'6 The meaning of thebubble pressure AP and the effective pore surface to volume ratio 8 isappreciated by consideration of FIGS. A and B depicting a liquid havinga surface tension 0' at pressure P, in a pore of radius R which isterminated, as at the surface of a capillary material, in theillustration of FIG. A, or which has a vapor bubble in the pore as inthe illustration of FIG. B. In both cases the pore is assumed to havebeen evacuated prior to filling with liquid so that no gasses arepresent except for the vapor from the liquid. The liquid is assumed towet the pore wall with a zero contact angle and the pore, liquid, vaporand surroundings are all at temperature T. The liquid has a vaporpressure P, at temperature T so that vapor at this pressure fills allspaces not occupied by the liquid.

It is obvious that if the pressure in the liquid P, is greater than thevapor pressure P,, the liquid will flow to the right in FIG. A and willcollapse the vapor bubble in FIG. B, in both cases replacing the vaporand forcing it to condense. What is less obvious is that the liquid willremain in the capillary tube of FIG. A and the vapor bubble'willcollapse in FIG. B even when the vapor pressure P, is greater than theliquid pressure P, so long as the sum of the liquid pressure P and thebubble pressure P is greater than the vapor pressure P,. This is due tothe pressure exerted by the surface tension 0' in the curved where R andR are the, principal radii of the curvature of the interface. For acircular pore as illustrated in FIGS. A and B, R,=R R the radius of thespherical interface.

The bubble pressure P5 is the maximum pressure that can be supportedacross the interface In a pore of radius R, R ,,=R ,,,,=R, so that thebubble pressure, P =2o'/R. In an infinite plane slot between parallelsurfaces spaced apart a distance b, R =bl2 and R equals infinity so thatthe bubble pressure P =2o'/b.

The surface to volume ratio 8 of the circular pore is 8 g 21rR/1rR=2/R.The surface to volume ratio of the infinite slot is 8,,,,,=2/b. Thus, inequation 2, P =08 and P c-8. The relation of equation 2 is valid, ingeneral, for most uniform rounded cross sections without sharp cornersor reverse curved regions. Even in structures not fitting theseassumptions, an effective pore surface to volume ratio 8 defined as 8=P/o' is useful. Both P and 0' are easily measured and 8 is dependent onlyon the shape and size of the pores, that is, calculation of 8 frommeasurements of P using fluids of different a will always result in thesame value of 8 for a given capillary material.

Since the liquid-vapor interface can support a pressure differential upto the bubble pressure P508, a pore will remain filled with liquid andany bubble formed in it will collapse so long as P +P,, P,, or P, +o-8P,,. This is the prime condition for stable existence of liquid in thepores.

The heat flow per unit area I-l/A through a porous matrix of heatconductivity k with a temperature gradient dt/dx is d T k d T H/A*kM dx(k dx (3) where k is the heat conductivity of the solid substance out ofwhich the capillary matrix is constructed.

The parameters 1 0' and k are properties of the materials used in aparticular embodiment, while the parameters a, 8 and k /k are propertiesof the microstructure of the capillary matrix. The first set ofparameters is determined by the choice of materials employed in aparticular example. It is, however, feasible to determine that matrixstructure giving the best possible combination of a, 8 and wherenecessary k lk for a particular example chosen. In order to approachthis analysis on a stepwise basis, the matrix structure for a wick willbe analyzed first since it does not involve the additional complicatingfactor of heat conduction as is present in a vaporizer.

The ideal wick matrix, as mentioned hereinabove, comprises a bundle ofhexagonal tubes with infmitesimally thin walls forming a honeycombstructure. The fluid conductivity of each hexagonal tube isapproximately the same as that of a circular tube of the same area, sothat for this analysis the matrix will be treated as a bundle ofcircular tubes of radius R having a total internal area A equal to thecross-sectional area of the matrix. It might be noted that a hexagonaltube has only about 5 percent more actual wall area and about 2 56percent more effective wall area for determining the effective matrixpore surface to volume ratio 8 so that this is not a damagingassumptiomThe ratio of wall area to volume 8 of a circular pore is Itshould be noted that the ratio 8 for a bundle of N pores is the same asthat for a single pore since both the surface and volume (actualcross-sectional area) are multiplied by N.

The fluid conductivity at of the matrix for a fluid of unit viscosity isthe fluid conduction of a single pore divided by the area q h Pa s dP 2dP 1rR H WK 1 d x 8 Optimally both the fluid conductivity at and theeffective pore surface to volume 8 should be as great as possible;however, there is a limitationindependent of the radius of the pore thatWRWLILEZ For other microstructures than the thin-wall hexagonal tubesor, is always less than 1. Thus, while the wick matrix microstructureemciency is independent of a change of scale or size, that is e, willremain the same if the entire matrix is uniformly expanded or shrunk,this is not true of the surface to volume ratio 8 or the fluidconductivity 01. Thus, within the limit imposed by the wick matrixefiiciency 2,, remaining constant, it is possible to optimize the fluidconductivity 1: and surface to volume ratio 8 for any given matrixmicrogeometry by simply changing the scale since if x is an arbitrarydimension in an arbitrary microgcometry, then 8 equal C and at equal C 4where C, and C, are constants dependent only on the shape andindependent of the size 1:.

To illustrate the usefulness of the wick matrix efliciency e and how theparameters introduced so far determine the operation of a simple type ofheat pipe, the maximum heat flux capacity of such a heat pipe isdetermined. it is assumed that this heat pipe comprises a cylinder ofcapillary material of length L and cross section A with the bottomsurface just touching the surface of a pool of liquid of density p,surface tension and viscosity 1; which wets the capillary material witha zero contact angle. The top end of the cylinder lies a distance zabove the liquid surface (the cylinder is not necessarily vertical), andthe top end is radiantly heated so as to evaporate liquid from thissurface. All of this above structure is enclosed and purged so as to befree from any residual gases other than vapor of the liquid. Thepressure differential AP necessary to drive a flow F of fluid throughthe capillary material is APF 0A 7 and the pressure difi'erential APnecessary to overcome the gravitational head is Thus, the pressuredifferential AP that must be provided by the capillary forces is orsolving for the fluid flow rate F The maximum heat flux capacity of theheat pipe given a fluid heat of evaporation per unit volume of h isAssuming that the scale factor of the microgeometry of the matrix, whichhas an eErciency factor of c can be adjusted so as to maximize the heatflux H, since The value of the effective matrix pore surface to volumeratio 8 that maximizes the heat flux is found by setting dH/d'o equal 0.Finding bility H of the heat pipe. This is true not only in thisparticular example but in all cases where the capillary matrix actspurely as a wick for liquid flow, that is where it does not alsotransmit heat as part of its function. In the more general case where apressure differential AP must also be present for the circulation ofvapor within the heat pipe and possibly for circulating liquid in aportion extemal to the wick equation 14 is readily modified by adding APto the pgz term and letting AP,=AP +pgz, that is AP, is set equal to allof the extemal" pressure differences in the total fluid flow system,that is, all the pressure difierences except that required to overcomethe viscous drag in driving the liquid through the capillary matrixbeing considered. Then sTLAP. A 81 LAP 15 where 6,, =2AP /a. Since AP isgenerally a function of the vapor flow rate in the heat pipe and henceof the total heat flux, 15) is generally at least cubic in terms of theheat flux H, and the equation is most easily solved by numerical approximation rather than an analytic solution.

In the example just set forth, the liquid is vaporized from a surfaceopposite to the surface from which liquid is introduced to the wick. Analmost identical set of formulas results when the liquid is evaporatedfrom a side adjacent to the side where the liquid is introduced. FIG. 1illustrates a rectangular slab of capillary material with a width w andthickness t with w much greater than t and of indefinite length x.Liquid is introduced to the capillary material at the side having across sectional area tx and evaporated from the adjacent face having anarea wx, with the heat for vaporization being supplied to that face byradiant heating, for example. Thus, the maximum heat flux capability ofthe capillary material is HIDBX:

e htx 11105 When the eflective matrix pore surface to volume ratio 6 isselected to maximize the heat flux l-l max The function of the capillarymatrix is considerably different and somewhat more complicated when itis used as a conductively heated vaporizer rather than merely a simplewick since it must conduct both liquid and heat to the surface at whichthe liquid vaporizes so that the heat conductivity k of the matrixbecomes important. The surface tension forces between the liquid andmatrix must also support an additional pressure differential sinceportions of the matrix must be hotter and therefore the liquid vaporpressure in that portion greater than at a surface of vaporization. Thisis so for causing heat to flow to the surface of vaporization from thehotter region.

The only exception to this situation in a conductively heated vaporizer(that is, where the heat is transferred to the surface of vaporizationby conduction through the porous matrix and not by radiation orconvection) is when the surface of vaporization lies within thecapillary matrix between the heat and liquid sources. Under thesecircumstances, the vapor must escape from the capillary matrix byflowing through the pores in the matrix. This is a severe restriction onoperation as can be seen from the situation for water and water vapor at100 C where the mass flow rate of liquid water through a porous materialunder a selected pressure differential is about seventy times the massflow rate of water vapor through the same porous material under the samepressure differential. Thus is due to the much lower density of thewater vapor and thus the much higher flow velocity required to give thesame mass flow rate. The higher velocity far outweighs the effect of thelower viscosity of the vapor. Also, when the varmrization surface isbetween the heat source and the liquid source, the vapor produced may belargely blocked from escaping since it cannot flow through the samecapillaries occupied by the liquid. Hence, such an arrangement is notpractical structure for most situations. The examples set forth hereinare thus limited to structures where the surface of vaporizationcoincides with a surface of the porous matrix from which vapor mayeasily escape.

The simplest structure of this type comprises a substantiallyrectangular strip of capillary material of width w, thickness t andlength x which is fed with liquid through one face of area tx and heatedthrough one of the adjacent two faces having area wx. So far this isexactly the same as the case of the radiantly heated vaporizer striphereinabove described. In this latter case, however, the face wx isheated conductively through an impervious wall adjacent the face so thatvapor can no longer escape from that face but must escape from theopposite face as illustrated in FIG. 2.

A heat flux density H/A,=H/wx must pass through the strip normal to theface wx requiring a temperature gradient :11 ldy to cause heat flow,where The increase in vapor pressure AP, of the liquid between y= and ywhere y is the distance of the liquid-vapor interface from the free orvaporizing surface, is

d l" g y(dPt-/dT) d T a: l mb 2 1 The pressure gradient dP /dz in theliquid necessary to force the liquid through the matrix is The pressurein the matrix P is assumed to be a function of only z, that is theliquid flows only in the x direction. This is an approximation thatbecomes exact as the thickness decreases and width increases, that isr/w approaches zero.

The position y of the internal liquid-vapor interface for maximum heatflux is determined from equations and 22 since along the liquid-vaporinterface AP,,AP equals a constant. AT maximum heat flux H the liquidfilled portion of the capillary will just reach the end in a thin wedge.That is, the thickness y=0 at a distance z=w. This can be seen since ify 0 at the far edge of the matrix, then the heat flux could be increasedand, on the other hand, if y 0 at the far edge, then the heat fluxdensity must be decreased until the liquid extends across the entirecapillary matrix. Also dP /dz=dP,/dz. Therefore equating the derivitivesand solving for dy/dz When z=w, and 'wz+C=0, so that C -w /2,

The thickness y is then substituted back into equations 21 and dp 1/2 Whe k 2 Human hEwkM 1/2 P 1/2 A P a: dP =zr dP 1- 2 W 6 21 dT 0'6Equation 31 is true when the maximum value of y is equal to or less thanthe thickness 1 so that g 1/2 a g k 1/2 tZw h d'Pv or he QB, g1

W dT 34 It should be recognized that equations 33 and 34 areapproximations since the pressure in the matrix P is assumed to be afunction of 2 only, that is dP /dy=0. This is strictly true only whenthe liquid flow is normal to the heat flow at all points, which is thecase when the ratio of thickness to width t/w is very small. Forstructures where t/w is relatively large, geometrical correction factorsG and G; can be added to the equations. These correction factors aredependent only on the geometry and approach 1 when the liquid flowbecomes nearly perpendicular to the heat flow. Generally G and G must becalculated numerically or by analog means.

Equations similar to Equations 33 and 34 can be derived for any stripgeometry (at leastwhere the faces are not extremely concave) that can beformed from therectangular geometry max hereinabove described bvstretching or shrinkin the strin in such a manner that they arestretched or shrunk in the same amount in the y direction as in the zdirection at each point. The same limitation that the heat and liquidflows are substantially normal to each other still exists. The resultingequations, including the geometric factors, are

replaces l/, in equation 34.

A truncated wedge geometry formed from a sector of a flat circular diskof radius R and truncated at radius 1R with liquid applied to thearcuate face and the heating and vaporizing surfaces being the tworadial faces forms a particularly useful example of the generalizedgeometry. Here letting dy=d0 and 37A and where l]! is the mean value ofUr. Thus equation 35 remains unchanged and equation 36 becomes Gains R 1M The general equation 35 for maximum heat flux as limited in range byequation 36, is applicable to substantially all of the structures ofinterest. Equation 36 limits the ratio of effective matrix pore surfaceto volume ratio 8 which can be considered and thus limits the pressuredifference AP, external to the vaporizer against which the vaporizer canoperate since AP 0'6. This is not a significant limitation in mostvaporizer designs, such as in heat pipes or surfaces exposed directly tobulk liquid, since in these cases AP, is small.

A general equation similar to equation 36 for the situation Thegeometrical factors G and G, in these equations, while both approachingl as the liquid flow direction approaches solely dependent upon thegeometry but are also somewhat dependent on C (equation 40A). This istrue since the internal liquid-vapor interface position for maximum heatflux varies with C in this case, thus changing the localized flowgeometry. Thus is to be contrasted with equations 35 and 36 wherein theflow geometry for maximum heat flux is independent of any othervariables.

When C is small the values for H /x given by equation 41 closelyapproach those given by Equation 35. For the case 0 when C is verylarge, the maximum heat flux H l X ap dT 2 411AP,w(;) 43 which is thesame as Equation l9 for the radiantly heated evaporator where 6 /656,;and (HI) =1/t. This is to be expected since the liquid fills almost theentire porous matrix at high values of C.

Due to its relative simplicity, Equation 35 for maximum heat flux H Jxis employed hereinafter although equation 41 can be substituted in itsplace as desired in applications involving high values ofAP, or C.

The temperature drop AT, across the vaporizer capillary matrix betweenthe heated surface and the surface where liquid is vaporizing is Thistemperature drop is generally quite small, being on the order of only afraction up to several degrees centigrade, with the higher valuescorresponding to higher values of heat flux H/x and external pressuredrop AP...

In order to provide comparisons of performance parameters in a vaporizersurface structure, an efiiciency factor 65; for the matrix structure isdesirable. The efficiency a of a selected capillary matrixmicrostructure is defined as the ratio of the maximum heat flux H /x fora vaporizer constructed with the capillary matrix microstructurerelative to that of a vaporizer constructed with an ideal capillarymatrix microstructure. Both matrices are made of the same material andall the other conditions are identical except for the microstructure. Toimplement this an ideal matrix microstructure geometry must be selected.

In equation 35, the only parameters dependent on the capillary matrixmicrostructure are 6., and k Further e is dependent only on themicrostructure, while It is divisible into two factors, the heatconductivity k of the material the matrix is constructed of, and therelative heat conductivity of the matrix k /k which is dependent only onthe matrix structure. Thus, the ideal geometry is one that maximizes thefactor e k /k which is proportional to a6 k /k A suitable standard orideal" vaporizer microstructure is one composed of parallel sheets ofheat conducting material of thickness a" mutually spaced apart to formchannels therebetween of width b. For this structure,

and then kM) =1 2 5 mad! The evaporator matrix efliciency factor 6 isthen defined so as to equal l for the ideal" vaporizer matrixmicrostructure 12ak tawny T, k (54) Using equation 54, equations 35 and36 for maximum heat flux are rewritten in tenns of the efficiency factore; as

It is instructive to compare the maximum possible heat flux per unitlength of the radiantly heated vaporizer (equation 19) with that of theconductively heated vaporizer, assuming ideal situations where e,,=e =l;(F -=1; O t/=1 in conduct dPv 1/2 3h i A x 7 5 Substituting theappropriate numerical values for water and copper at 100 C.

nsu

2: "dim, 5.9X 10 3X2.16X-10 X3.68 10 (H,,,,,,) "2(AP,) 2.8X10' X3.8

a: conduction equivalent to a pressure head of from about 10 to 100centimeters of water and T fl 440 to 44 respectively Z )eonductic nThus, except at unusually high gravity heads or other external pressuredrops, a conductively heated vaporizer strip is limited to far lowerheat flux density than a comparable radiantly heated vaporizer strip.Since in most cases of interest, the object of the heat transfer surfaceis to remove heat from a hot surface or body where the heat radiated isfar less than the heat it is necessary to remove, the heat usually'mustbe transferred to the vaporizer conductively. Thus, any means forovercoming the heat flux limitation of the conductively heated vaporizeris desirable as leading to greatly increased utility for the structure.

The new type of conductively heated capillary vaporizer surface,described herein, avoids the heat flux limitation caused by overheatingof the porous matrix and the consequent displacement of the liquid fromit. This is accomplished by keeping the heat from having to be conductedvery far through the matrix and particularly from having to be conductedacross the principal width of the matrix supplying the liquid. On theother hand, as pointed out hereinabove, it is desirable to have thesurface of vaporization coincide with a surface of the porous matrixfrom which the vapor may escape with minimum restriction.

It is, therefore, an important feature of the capillary vaporizersurface structure described herein that passages are provided in thevaporizer structure which form, or connect to, cavities forming surfacesin the capillary matrix material near the heat source where the liquidis vaporized and from which the resulting vapor may escape from thecapillary matrix. The passages may lie in either or both the capillarymatrix material or the material between the heat source and thecapillary material so long as the passages expose. or connect tocavities 0 exposing, surfaces of capillary material and provide for theescape of vapor formed at these exposed surfaces.

In a preferred embodiment for large areas and high heat flux densities,there are a hierarchy of vapor passages wherein very numerous, verysmall passages feed vapor into larger, less numerous passages which may,in turn, feed into a few rather large passages. Capillary arrays of thistype simultaneously minimize the pressure differential necessary tocause vapor flow from the array and also minimize the amount ofcapillary material removed or deleted to form the passages and thus notavailable for liquid transport. This array also allows a network of veryclosely spaced passages which form, or are connected to, regional areasof vaporization, to be placed adjacent the heated surface of thecapillary material.

Elaborate arrays having a multiple step hierarchy of passages are notnecessary in many heat transfer situations. Thus, for example, aninexpensive very high heat flux boiling" surface which is in contactwith bulk liquid with which vapor may freely mix, for example, simplyprovides a layer of capillary material perforated by a large number ofslits or holes reaching from the bulk liquid surface through or almostthrough the capillary material to the heat source surface. In such aheat transfer surface structure, the gross heat and liquid flows are inopposed direction, although within the capillary material, heat flow issubstantially normal to liquid flow.

FIG. 3 illustrates in perspective a capillary vaporizer surfaceconstructed according to principles of this invention. As illustrated inthis embodiment, there is a heat source wall 11, preferably of arelatively high thermal conductivity metal. The heat source wall may,for example, be a wall separating heat transfer fluids in a heatexchanger, or may be a surface portion of a heat producing electroniccomponent or the like.

In intimate thermal contact with the heat source wall 11 is a channeledlayer of porous material having a high efificiency 6 It is to beunderstood that, as used herein, the term porous is meant to includecapillary materials whether the capillary liquid conduits are strictlypores or not. Thus, for example, a solid surface with grooves or afolded foil might be used instead of a porous matrix in someembodiments. Also, in describing a capillary as having a larger orsmaller pore size, what is specifically meant is that the capillarystructure has a lower or higher value, respectively, of the effectivepore surface to volume ratio 8, rather than any particular dimension ofthe capillary structure. The channeled layer adjacent the wall 1 1 canbe considered to be a plurality of parallel spaced apart strips 12 ofporous material joined to the heat source wall 11. Preferably, thestrips 12 are flared in the portion adjacent the heat source wall sothat a substantially continuous layer of porous material is provided onthe heat source wall with the thickness of the .layer increasing from avery small thickness intermediate the strips to' an appreciablethickness in the principal portion of the strip. Each of the strips 12is overlaid by a strip 13- of similar size having'a larger pore size(that is, lower 6) than the porous material forming the strips 12. Thelarger pore size material 13 acts as a temporary reservoir of liquidduring operation of the capillary vaporizer surface.

In a typical embodiment, the surface illustrated in FIG. 3 is employedas a so-called boiling heat transfer surface where the surface is incontact with bulk liquid that is free to mix with vapor produced at thesurface, that is, the liquid reaching the porous capillary vaporizer isnot previously confined within a capillary wick adjacent the vaporizer.It should be noted that this capillary vaporizer surface is not employedexclusively in a situation where the surface is immersed in a body ofliquid but may be employed in a situation where the surface isintermittently wetted by a liquid and intermittently exposed only tovapor as may occur in a high flow rate heat exchanger. in thissituation, the period of time that the surface is not wetted by liquidis extremely short. Therefore, a relatively small strip 13 of largerpore material can act as a temporary liquid reservoir of sufficientcapacity to provide liquid to the vaporizer during the intervals thatthe local surface is not wetted.

During operation of the heat transfer surface illustrated in FIG. 3 heatflows through the wall ill and into the porous vaporizer strips 12 attheir wide portions. Liquid contacts the vaporizer strips primarily inthe larger pore portion 13, and due to surface tension forces flowsthrough the coarser pored material to the finer pored vaporizer strips12. Heat also flows through the vaporizer strips 12 and vaporization ofliquid occurs primarily at the very large number of regional areasformed by the sloping bottom surfaces M in the channels between thestrips. These surfaces are nearest heat source wall, so that the heatneed flow only through a relatively short path of vaporizer, and thesurfaces are also adjacent the channels so that the vapor formed canfreely escape from the capillary vaporizer surface without passingthrough the pores.

It should be observed that the width and height of the strips of porousmaterial on the heat transfer surface are quite small, and, for example,may be about 0.002 to about 0.04 inch. The smaller sizes are preferablefrom the point of view of enhanced heat transfer since a greater totallength of strips can be accommodated on a surface of given area. Aspointed out hereinabove, a greater heat transfer rate can be obtainedsince the rate is dependent on a total length of strip and is notaffected by the size of the strip, and close packing of small stripsgives higher heat flux density than fewer longer strips. The smallerstrips are, however, somewhat more difficult to fabricate, and in orderto avoid expense, some sacrifice may be made in heat transfercharacteristics and large strips may be employed.

A porous capillary vaporizer, as illustrated in FIG. 3, is

readily made by bonding a layer of high efiiciency porous material tothe heat source wall, preferably by diffusion bonding to avoid pluggingpores, and in a similar manner a layer of coarser pored material isbonded on the finer material. The surface can then be grooved to producethe structure illustrated in FIG. 3. It will also be apparent that thetwo-layer structure is not require in all situations, and a lessexpensive structure can be prepared with a single channeled layer on theheat source wall.

FIGS. 4 and 5 illustrate in two perpendicular cross sections a compoundcapillary vaporizer surface incorporating principles of this invention.As illustrated in this embodiment there is a heat source wall 21, which,as before, is preferably a high thermal conductivity metal through whichheat flows into the capillary vaporizer from some heat source (notshown). Immediately adjacent and in intimate thermal contact with theheat source wall are a plurality of capillary vaporizer strips 22 formedof a porous material having a high efficiency 6 The strips are placed inclose proximity to each other so that there are a plurality of channelstherebetween, and the width and height of the strips 22 is preferably inthe same order as those strips illustrated in FIG. 3

The strips 22 may have a cross section such as illustrated in FIG. 3, ormay merely be rectangular strips either in direct contact with the heatsource wall or with a thin layer (a few mils) of additional porousmaterial uniformly covering the heat source wall.

Liquid is delivered to the vaporizer strips 22 in the compound vaporizersurface by somewhat larger strips 23 running approximately perpendicularto the smaller strips 22 on the heat transfer surface. The larger strips23 are spaced apart to leave channels 24 therebetween that are largerthan the channels between the smallest strips 22. Overlying the largerstrips 23 are bars 26 running approximately perpendicular to the largerstrips 23 and parallel to the heat source wall 21. The bars 26 aremutually spaced apart to leave channels 27 therebetween that areappreciably larger than the channels 24 between the larger strips 23.The larger strips 23 and bars 26 are made of a porous material having ahigh wicking efiiciency 6,, and would normally have larger pores (thatis, lower 8) than the smallest strips 22 immediately adjacent the heatsource wall.

As an example of the scale involved, the bars 26 may in one example beabout 0.016 inch wide and 0.032 inch high in a direction normal to theheat source wall. The size of the bars can, in general, range upwardlyfrom these values, It might also be noted that the strips 23 and bars 26being made of the same material, can be made in a single operationwherein one ridged die is pressed towards another ridged die with theridges of the two dies perpendicular to each other. This combination ofbars and strips can also be made from a slab of porous material bycutting the channels 24 in one direction and the channels 27 in oppositedirection sufficiently deeply to intersect.

Overlying the bars 26 and in intimate contact therewith are still largerbars 28 running parallel to the heat source wall, and approximatelyperpendicular to the smaller bars 26. The larger bars 28 are mutuallyspaced apart to form channels 29 therebetween that are larger thffthechannelsfi betweiifhe smaller bars 26. These larger bars are preferablymade with an even larger pore diameter (that is, lower 8) than thesmaller bars 26. These larger bars act as temporary liquid reservoirsduring operation of the capillary vaporizer.

In operation, liquid contacts the larger bars 28 either as bulk liquidin intermittent contact with the exposed faces of the bars or in someembodiments by an additional wick (not shown) bringing liquid thereto.The liquid flows from the larger bars 28 under surface tension forces,as hereinabove described, into the smaller bars 26 and thence into thestrips 23 closer to the heat source wall. These bars and strips do nothave any substantial heat flow therethrough and behave as wicks forconducting liquid to the smallest strips 22 in contact with the heatsource wall 21. Thus, the larger strips and bars service as a liquiddistribution system for bringing liquid to the smallest strips 22 whereit flows to the interfaces between the strips 22 and the channelsbetween them, from which interfaces it evaporates into the channels.These interfaces are, thus, regional areas of vaporization formed" bythe channels. In some embodiments a thin layer of porous material may beprovided over the entire heat source wall 21. Such a layer makes littledifference in the maximum heat flux capability of the vaporizer but canappreciably reduce the temperature across it. The regional area ofvaporization as used herein includes the interfaces of adjacent strips(in this embodiment) on opposite edges of the channel and the porouslayer, if any, adjacent the heat source surface.

The vapor that is formed in the channels between the smallest strips 22flows into the channels 24 between the larger strips 23. The vaporeither flows directly into the larger channels or may pass for a shortdistance parallel to the heat source wall before reaching one of thelarger channels 24. The distance that the vapor need flow through thesmall channels between the smallest strips 22 is quite short, so thatthere is as little as possible flow resistance in that short passage.

The vapor within the passages 24 between the larger strips then flowsinto the still larger channels 27 between the bars 2 6. The vapor flowsthrough the larger channels into the still larger channels 29 betweenthe largest bars 28 and thence escapes from the capillary vaporizer.Thus, it will be seen that there is a liquid flow path through thesuccessively smaller and more numerous bars 28 and 26 and the strips 23to the smallest and most numerous strips 22. The vapor flowscountercurrent to the liquid in a parallel path from the smallest andmost numerous channels through the succesively larger and less numerouschannels 24, 27 and 29. The liquid flow path is thus separated from thevapor flow path, and there is no interference between the counterflowingfluids.

The liquid flows through porous materials having successively smallerpore sizes (larger 8) so as to support the increased pressuredifferentials due to viscous drag and due to the increasing vaporpressure from the increasing temperature. As the cross sections becomesmaller, the number of flow paths increases so that the total areaavailable for liquid flow remains substantially constant throughout thestructure in any plane parallel to the heat source wall and the lengthof the path over which the liquid flows also decreases so as to minimizethe liquid pressure drop. In a similar manner, the vapor flow throughsuccessively smaller numbers of successively larger channels to escapefrom the structure, and the overall area available for vapor flowremains substantially constant in all planes parallel to the heartsource wall. It is generally preferred that the cross-sectional areaavailable for vapor flow and liquid flow be about equal since thisyields a near optimum overall heat transfer efficiency for the capillaryvaporizer over a broad range of operating conditions.

Both of the structures hereinabove described and illustrated in FIG. 3,and in FIGS. 4 and 5 can be considered as vented capillary vaporizerssince holes or passages are provided for escape or venting of vapor fromthe capillary matrix. Determination of the performance characteristicsof a vented capillary vaporizer divides into two parts l) the vaporizingsurface structure. including the heated surface and the immediatelyadjacent capillary material and passages, that is, the region where heatconduction through the matrix and evaporation from the passage wallstake place, and (2) the liquid distribution and vapor collectionmanifolds wherein fluid flow occurs and heat flow can be ignored.

Considering the second part first, the liquid may be distributeddirectly to the capillary material forming the vaporizing surfacestructure, that is, the layer of capillary material immediately adjacentthe heat source surface, as in the simple boiling" surface structurehereinabove described and illustrated in FIG. 3, wherein the vapor islikewise directly collected and discharged into the same region thatsupplies the liquid so that the liquid and vapor are comingled. In thistype of structure, any heat flux limitations lie either in thevaporizing surface structure, or in "mechanical interference between theincoming liquid and outgoing vapor, which is best determined empiricallysince the general mathematical solution of the dynamic situation withoutliquid or vapor manifolds is exceedingly complex.

In a structure as hereinabove described and illustrated in FIGS. 4 and5, the liquid is distributed to the vaporizing surface structure bycapillary material which also contains passages for the vapor to escape.The liquid flow is calculated in the same manner as hereinabovedescribed wherein equations ll and 15, orl'8 and 19 were derivedutilizing the fluid parameters: density p, heat of vaporization per unitvolume h, surface tension and viscosity 1 and the matrix parameters:fluid conductivity a, pore surface to volume ratio 8, and wick matrixmicrostructure efficiency 6,; and pressure differences external to thevaporizer, such as AP, and AP Generally the same calculation procedureis employed. The pressure drop across the matrix required to force thedesired fluid flow rate is calculated in terms of the fluid properties,the external pressure differences, and a and 6. Then the equationrx---e,,,/215 is used to eliminate a in favor of the efficiency factor cwhich is not dependent on scale or size. The external pressuredifierences are added and the result set equal to 0-8 and solved for thefluid flow rate. The expression for the flow rate is then differentiatedwith respect to 8, set equal to zero and solved for 8 so as to get thevalue of 8 giving the maximum flow rate. A capillary matrix structure isthen selected having a 8 as close as possible to that calculated andefficiency factor 6,, as high as possible and the actual values of 8 and6,, entered in the equation for the flow rate, thus giving the solution.

The pressure drop due to vapor flow in the passages is calculated usingstandard techniques for fluid flow. Care must be taken in thesecalculations since the vapor flow often shifts from laminar in thesmaller passages to turbulent flow in the larger passages. The pressuredrop thus calculated is included as one of the external" pressure dropsin the liquid flow calculations. In finding the vaporizing surfacecharacteristics, it is important to note that the use of multiplepassages to vent a capillary vaporizer does not alter the basicequations 55 and 56 giving the maximum heat flux per unit length, andthe maximum values of 08 of P, for a conductively heated capillaryvaporizer strip. Multiple vapor passages in the vaporizing surfacestructure, however, permits the use of as large a number of vaporizerstrips per unit length as is necessary to obtain the desired heat fluxdensity. Thus, for example, a passage through a porous material adjacentand parallel to the heat source surface creates two vaporizer strips,one on each side thereof, each capable of the heat flux per unit lengthgiven in equation 55. Thus, increasing the number of passages adjacentthe heat source surface, increases the maximum available heat fluxproportionately until a limit is reached wherein the resistance to vaporflow becomes excessive due to the small size of passages.

The increase in heat flux occurs since the flux is not dependent onscale, i.e., size, but only on shape.

In the same manner as for parallel strips, a hole normal to the heatsource surface penetrating through or nearly through the capillarymatrix produces a circular vaporizer strip around the hole where itapproaches the heat source surface and such a hole also serves toincrease the heat flux capability of the surface, although due to theunusual shape of the strip, quantitative determination of the increaseis mathe matically more difficult.

Calculation of the maximum heat flux per unit heat source area H /A, forany selected vaporizer surface structure is straightforward when theevaporator strip shape efficiency factor G, and the fluid and matrixparameters are known, since tnax max) hks 1/2 APX Yr Q 1 d6 where S isthe total length of the vaporizer strips per unit heat source area.Thus, for example, if there are n, passages per unit distance, formingan equal number of regional areas of vaporization, each of which iscomposed of two vaporizer strips, side by side and parallel and adjacentto the heat source surface, then S=2n, and

value for the factors G, and 6,, independent of any other facv tor.Thus, all scaling and size laws are implicit in equation 55 and do notrequire a knowledge of the geometrical factors G, and G, which arerequired only when the numerical value of the maximum heat flux must becalculated and not when the effect of various parametric changes isrequired. Generally, it is preferred to use structures having as high avalue of the geometrical shape factor G as possible to maximize the heatflux per unit area H /A Therefore, structures described herein areselected to have an estimated value for the geometrical factor in therange of about 0.5 to l'.0.

In order to illustrate the magiitude of the heat flux densities involvedin the improved vented capillary vaporizer, constants appropriate towater in a copper matrix at 100 C. are employed in equation 61. First,the product of surface tension and efiective pore surface to volumeratio 8 is found from equation 56 Further assuming reasonable values forthe geometric constant G I, efiiciency of the vaporizer matrix e,-,-=0.2thermal conductivity ratio k lk =4 and a geometry w {l/r}=l then theproduct 08; 1 8X 1 0 dynes/Crh. which is slightly over one atmosphere.It might be noted that for the ideal" case, where e =l and k,/k,,;=2,then the product a'6==2.94 l0 dynes/cm. Usual operating conditions for avaporizer have external" pressure gradients AP, values ranging fromabout 10 to about 2X10 dynes/cm., that is about 10 centimeters to 2meters equivalent water head. Higher values of AP can be dealt with asindicated in equations 39 through 42.

Now finding the maximum heat flux per unit area H /A, for water in acopper matrix at 100 C.

Further assuming that G,=0.8, 5=O.2, and AP,=o-6/4, then H /A,,=34.5n,watts/cm? and AP,=3.0 10 dynes/cmF.

Theoretically, the number of vapor passages, or regional areas ofvaporization, per unit length n, can be made as large as desired, andonly limit on H A lies in the fact that the external pressure AP,contains a term equal to the pressure drop due to the vapor flow out ofthe vaporizer and through the rest of the system, if any. This term isusually one to 10 times the maximum vapor velocity pressure p, /2 formost vaporizer designs with the simple structures of FIGS. 3, d and 5being close to one. Thus the external pressure drop AP, increases withincreasing heat flux H /A thereby decreasing the term (as-AL). As apractical matter there are also other considerations with very largevalues of n, since construction of the vented vaporizer becomesincreasingly difficult as the passages or channels become smaller, andalso higher values of the efifective surface to volume ratio meansmaller pores which are clogged more easily by deposits left when theliquid evaporates.

Thus, for example, a vaporizing surface structure suitable for manyapplications would have a number of channels n, of about 40 percentimeter, that is, about 0.010 inch between channels. This gives amaximum heat flux per unit area of about 1,380 watts per squarecentimeter for water in a porous copper matrix at 100 C. A vaporizersurface structure for spot cooling a small area, such as an electronicdevice, may have as many as 200 channels per centimeter so that themaximum heat flux H /A, is in the order of 6,900 watts per squarecentimeter for water in a copper matrix at l00 C. These heat fluxdensities are more than an order magnitude above those that are usuallyavailable with pool boiling, for example, which has a maximum heat fluxof about watts per square centimeter for water at atmospheric pressure.MOst other vaporization or convective cooling systems have even lowerheat flux densities than pool boiling.

FIG. 6 illustrates in cross section a single vaporizer strip structuresimilar to that illustrated in FIG. 3. The structure includes a heatsource wall 31, which is impervious to liquid and vapor and throughwhich heat flows to the larger parallel face of a trapezoidal crowsection vaporizer strip 32 formed of relatively small pore, high thermalconductivity porous material. On the smaller parallel face of thetrapezoidal vaporizer strip 32 is a feeder strip 33 of relatively largerpore material in intimate contact with the smaller pore material of thevaporizer. The liquid (not shown) is at least intermittently in contactwith the feeder strip 33, and the liquid flows through the porous matrixto the sloping faces 34 of the trapezoidal strip as indicated by thesolid arrows. Vaporization of the liquid occurs principally at thesurfaces 34, and vapor escapes from the region between the strip and anadjacent similar strip as indicated by the dashed arrows.

The vaporizer strip structure illustrated in FIG. 6 essentially formstwo vaporizer strips in the sense that S is used in equation 60, or itcan be considered that the space on either side of the porous materialis combined to be equivalent to one passage. Equation 61 may thereforebe used to calculate the maximum heat flux density, with n, being thefrequency of vaporizer strip structures on the surface. The vaporizerstrip structure illustrated in FIG. 6 can be considered to be formed oftwo truncated sectors having liquid entering from one face, heatentering from a second face, and vapor escaping from a third face, withthe direction of heat flow and liquid flow being substantiallyperpendicular. Approximate calculations for such a structure indicate ageometrical shape factor G, of about 0.8 to 0.9 for this geometry.

FIG. 6 also shows by a dashed line 36 an internal liquidvapor interfacewithin the porous matrix of the vaporizer. This interface 36 arisessince the heat source wall is at a higher temperature than thevaporizing surface 34 in order for heat to flow from the wall to thesurface. When operating at high heat flux densities there is a regionnear the heat source, as indicated by the interface line, where thetemperature in the matrix is sufficiently high to drive liquid from thepores and replace it with vapor. At low heat flux, the difference intemperature across the vaporizer will be relatively low, and theinterface 36 will be relatively nearer the heat saurce wall or maydisappear when the matrix is completely liquid filled. When the heatflux is high, the temperature gradient will be higher, and the interfacewill be relatively nearer the vaporizer surfaces. Eventually there is amaximum continuous heat flux which cannot be exceeded without theinternal liquid-vapor interface moving so near the surface as to sochoke the liquid flow that insufficient liquid is supplied to replacethat vaporized, leading to a drived-out" condition and possibly damageto the structure. The heat transfer surface structure will accommodateany heat flux less than the maximum available.

FIG. 7 comprises a capillary vaporizer surface structure which can beconsidered to be a detail of a larger structure such as illustrated inFIGS. 4 and 5. As illustrated in FIG. 7, the heat source wall 21 has asmaller strip 22 in intimate thermal contact therewith, and the strip 22is also in contact with a larger feeder strip 23 of larger poredmaterial. As seen In this detail, liquid flows from the larger strip 23through the small vaporizer strip 22, which can for purposes ofcalculation be considered to be two connected vaporizer strips. Theliquid flow is indicated by solid arrows in FIG. 7 and vapor flow fromthe side faces 38 of the vaporizer strip 22 is indicated by dashedarrows.

As in FIG. 6, a vapor-liquid interface 39 occurs within the vaporizerstrip bounding the region where the temperature is high enough that onlyvapor is stable, and the lower temperature region where liquid is stablewithin the porous matrix. The three dimensional shape of the interface39 is complicated by the fact that liquid also flows through thevaporizer strip 22 in a direction along its length, that is, normal tothe plane of the paper in FIG. 7, and the interface is further from theheat source wall in the portion of the smaller strip 22 between thelarger feeder strip 23 and the wall, then it is beneath a channel 24(FIG. between a pair of feeder strips 23.

FIGS. 8, 9 and illustrate alternative detailed structures of a capillaryvaporizer surface structure incorporating principles of this inventionIn each of these figures, A single vaporizer strip structure andone-half of a channel on each side thereof is illustrated, and it willbe understood that similar parallel structures occur repetitively on theheat transfer surface.

As illustrated in FIG. 8, a heat source wall 41 is coated with acontinuous layer 42 of fine pored, relatively high thermal conductivityvaporizer material, from the opposite surface of which vaporizationoccurs as indicated by the dashed arrows. A larger pore size feederstrip 43 seen in cross section has a narrowed portion with a small face44 in contact with the vaporizer strip 42. Liquid flows from the feederstrip 43 to the vaporizer 42 as indicated by the solid arrows. Aninternal liquid vapor interface 46 occurs within the vaporizer 42 athigh heat fluxes and the maximum heat flux is obtained when theinterface just contacts the surface from which vaporization occurs atthe midpoint of the channel between adjacent feeders 43.

FIG. 9 illustrates a heat transfer surface structure that ismathematically substantially identical to the structure illustrated inFIG. 8. As illustrated in this embodiment, a heat source wall 47 isprovided with V-shaped grooves, the sides of which are lined with a finepored capillary material 48. A portion of the vaporizer at the crest ofridges in the heat source wall 47 is in contact with a relatively coarsepore feeder strip 49 running approximately perpendicular to the ridges.Liquid flows from the feeder 49 through the capillary material 48, asindicated by the solid arrows. Vapor escapes from the surface of thecapillary material into the Vshaped channels as shown by the dashedarrows. A vapor liquid interface 51 forms between the heat source wall47 and the surface of the capillary material from which vaporizationoccurs, in substantially the same manner as hereinabove illustrated inFIG. 8.

FIG. 10 illustrates a heat transfer surface structure particularlyuseful when the external pressure drop AP against which fluid fiow mustbe provided is relatively high. In this structure a heat source wall 53is provided with raised rectangular ridges 54 so as to define aplurality of rectangular channels 56 therebetween. The tops of theraised portions 54 of the wall are in intimate thermal contact with aporous matrix 57 that is supplied with a vaporizable liquid. The liquidflows through the porous matrix as indicated by' the solid arrows, andvaporization occurs at the face 53 of the matrix adjacent the channels56 as heat flows from the wall into the matrix. At high heat fluxes aliquid vapor interface 59 forms in the porous matrix as before. Theregions of porous matrix forming the vaporizer strips for purposes ofcalculating the maximum heat flux have unusually large thickness towidth ratios 1/w(l/ t) l and therefore will allow more area for fluidflow, which proves useful when the external pressure AP, is high so that8 must be highto support the pressure differential, thus forcing thefluid conductivity a to be low.

number of different configurations since the vapor passages only have topenetrate through or nearly through the capillary matrix materialbetween the heat source wall and the region occupied by combined liquidand vapor. Also, the passages in any capillary vaporizer may be ofalmost any shape, for example, grooves, slots or holes of any shape, andeven random structure such as a large pore matrix in which the largepores are the vapor passages, and the matrix is composed of materialthat is itself a fine pore matrix that carries the liquid. Layers ofsuch material, with the size of the larger pores, and possibly thesmaller pores, increasing with increasing distance from the heat sourcesurface may also be used as an inexpensive alternate to structures suchas illustrated in FIGS. 4 and 5.

Fabrication techniques are equally varied since the passages may beformed by bonding closely spaced strips, cylinders, grids, or evenirregular lumps of porous material to the heat source surface and toeach other. Another fabrication technique for a capillary vaporizersurface is to first coat the impervious heat source surface with acontinuous layer of porous material and then multiply crack the porousmatrix, for example, by stretching or bending the composite layers, orby further sintering of the porous material to induce additionalshrinkage.

Except for the simplest types of capillary vaporizer surfaces forcontact with bulk liquid, the vented capillary vaporizers comprise atleast a heat source surface, a vaporizer surface structure and a liquidfeed structure. Both of the latter being of capillary material forinducing liquid flow due to surface tension forces. It is important todistinguish these two structures since during operation of the vaporizerthe capillary material forming the surface structure transmits both heatand liquid while the capillary material forming the feed structure needtransmit only liquid, and in many situations it is desirable that it bea good thermal insulator. These functional differences between the twostructures are reflected in the efficiency factors for the twomaterials; that for the feed structure or wick being e =20z8 while theefficiency factor for the vaporizing surface structure is e =28(3a(k /kIn some situations, both the surface vaporizer structure and the feedstructure may be fabricated of the same porous material for lower cost;however, there may be some sacrifice in performance.

The vented capillary vaporizers provided in the practice of thisinvention may be categorized in steps of increasing complexity ofstructure and as to the extent to which the liquid and vapor flow arethereby isolated.

A simple heat transfer surface is hereinabove described and illustratedin FIG. 3, and comprises a layer of porous material bonded to a heatsource surface and perforated with passages penetrating directly intothe porous material to, or almost to, the heat source surface. The vaporproduced is thus vented directly into the same region carrying theliquid.

A compound heat transfer surface is hereinabove illustrated in FIGS. 4and 5, and is similar to the simple structure but with passages carryingat least a portion of the vapor a short distance parallel to the heattransfer surface before venting it into the region of mixed liquid andvapor. Thus, in a portion of the region adjacent the surface, theoutgoing vapor and incoming liquid have separate flow paths. Thecompound surface can be considered to be a simple heat transfer surfacecapped with feeder strips of porous material perpendicular to thevaporizer strips on the heat transfer surface forming liquid and vapormanifolds for separated countercurrent flow of the twofluids.

The capillary vapor strip designs illustrated in FIGS. 6 to 10 n aremerely exemplary of good vaporizer designs, and it should be apparent toone skilled in'the art that many-more shapes and combinations of shapesof porous materials can be used to form the porous matrix and thepassages to vent vapor from the surfaces where vaporizationoccurs.

A simple" capillary vaporizer surface structure as illustrated in FIG. 3(as contrasted to a compound boiling surface as illustrated in FIG. 4and slpermits a particularly large A third type of vented capillaryvaporizer is one where the liquid is fed thereto by a wick such as, forexample, in a heat pipe. In this structure, the-liquid is always-in somecapillary structure, either the wick, an intermediate feeder, or thevaporizer, and the flow paths of the liquid and vapor are therebyseparated. There is no need, however, to thermally or mechanicallyisolate the liquid filled wick from the vapor since the capillary forcesin the wick support the pressure difference between the liquid andvapor. If, for some reason, the vapor impinging on the wick issuperheated, liquid will merely evaporate from the surface of the wickto keep it cool. Thus, the actual vaporizer surface structure can bequite similar to the compound boiling surface, with the feed structurein contact with a wick. Such a wick-fed vaporizer structure isillustrated in FIGS. 1 I through 16, hereinafter described in greaterdetail.

A fourth type of vaporizer can be characterized as a capillary pump"vaporizer that is employed when the incoming liquid to the vaporizer isin bulk, and it is desired that the outgoing vapor be at a higherpressure than the incoming liquid. By bulk liquid is meant that theliquid is not flowing through a capillary structure suficiently finepored to support the desired pressure difl'erence. In a capillary pumpvaporizer, the incoming liquid must be both mechanically and thermallyiso lated from the outgoing vapor until the liquid flows into asufficiently fine pored matrix within or adjacent the vaporizer tosupport the required pressure difierential. If the incoming liquid isnot thermally isolated and, if necessary, cooled, its temperature andthus its vapor pressure will approach that of the outgoing vapor whichmay cause bubbles and eventual vapor lock in the incoming liquidpassage. Generally, at least part of both mechanical and thermalisolation is achieved by separating the bulk liquid input manifold fromthe complex vaporizer structure by a wicking layer of low thermalconductivity porous material with sufficiently fine pores to support thepressure difference between the liquid and vapor. A capillary pump typeof vaporizer is illustrated in FIGS. 17 to 19 and described hereinafterin greater detail.

F IGS. 11 to 14 show in cross-sectional views one of the third type ofvented capillary vaporizer wherein liquid is fed thereto by a wick. FIG.15 shows one element of this rather complex structure in perspective tohelp clarify the shape of the parts and FIG. 16 magnifies a smallportion of FIG. M for greater clarity. Such a wick-fed, vented vaporizerdesign is suitable for use in a multiply segmented heat pipe such asdescribed and illustrated in the aforementioned copending patentapplication entitled SEGMENTED HEAT PIPE. Such a vaporizer design isalso useful in ordinary heat pipes or the like where it is desired tomaximize the heat flux density while minimizing the temperature dropacross the capillary vaporizer by having a very short heat conductionpath.

As illustrated in this embodiment, a cylindrical impervious wall 61bounds the structure on the sides. A heat source wall or partition 62separates the vented capillary vaporizer from a heatsource 63 which isnot illustrated in great detail but which can be a somewhat similarsurface structure or some other surface adapted for vapor condensationas in a segmented heat pipe or may be a primary heat source. Immediatelyadjacent the heat source wall 62 is a thin evaporator surface layer 64of porous material having a high eficiency a (FIG. 16).

It is convenient in discussing the vented vaporizer illustrated in FIGS.11 through 16 to define orthogonal x and y coordinates lying in a planeparallel to the heat source wall 22. Thus, the cross sections of FIGS.ill and 13 are taken in planes parallel to the xy plane. The crosssection of FIG. 12 is taken in a plane parallel to the plane containingthe y-axis and the axis of the tube, and FIGS. M and K6 are crosssections taken in a plane parallel to the plane containing the x-axisand the axis of the tube.

Parallel to the heat source wall 62 and extending in the y direction area plurality of parallel strips 66 of porous material, each having across section substantially as illustrated in FIG. 8 with a narrow edgein contact with the porous layer 66 on the heat source wall and dividingthe free surface of the porous layer 64 into a number of regional areasof vaporization. The plurality of strips 66 defines a plurality ofintermediate channels 67 lying parallel to the heat source wall andextending in the y direction. The parallel strips 66 serve to deliverliquid to the porous material 64 on the heat source wall, and thechannels 67 serve to carry vapor away from the porous material as liquidis vaporized in the manner hereinabove described for FIG. 6.

Extending transversely to the strips 66 in contact with the porous layerare a plurality of larger strips 68 of rectangular cross section andextending parallel to the heat source wall in the x direction. Thespaces between the larger strips 68 form larger 1 direction channels 69,each of which is in fluid communication with a plurality of the smallery direction channels 67 so that vapor from a plurality of the smallerchannels feeds into the larger channels 69 during operation of thevaporizer. The larger strips 66 are in contact with the smaller strips66 and can even be formed integrally therewith of the same material.Both sets of strips are formed of a porous material having a highwicking efficiency c In a typical embodiment for a heat pipe aboutone-half inch diameter the smaller channels 67 may be present with afrequency in excess of about channels per inch, and the larger channels69 may be provided in the order of about 36 channels per inch.

In order to feed liquid to the larger strips 68, a capillary wickingstructure is provided which can be conveniently divided into aconventional cylindrical fluid source wick 71 of porous materialextending along the tubular wall 61 from some cooler region (not showntowards the other end of the heat pipe, and a somewhat more complexwicking structure between the fluid source wick 7i and the larger strips6d. The interior surface of the wick '71 defines a passage denominatedherein as a vapor way.

This intermediate wicking structure includes a ring 72 of porousmaterial and having an outside diameter approximately that of thetubular wall 61 of the heat pipe, At the end of the ring in contact withthe porous wick 71, the inside surface 74 of the ring 72 is in the formof an elliptical cylinder having the major axis of the ellipse in the xdirection and the minor axis in the y direction as seen in the crosssection of FIG. 11. In order to mate with this elliptical inside shape,the inside of the wick 71 is provided with a pair of beveled portions 73so as to provide an elliptical cross section at the end of the wick incontact with the end of the ring 72.

The elliptical internal surface 74 of the ring 72 extends the fulllength of the ring at the major axis, that is, in a plane in the xdirection as seen in FIG. 14. The inside surface of the ring is alsoprovided with a beveled region 76 which can be considered as the surfaceof an elliptical cone, that is, a cone having an elliptical base and anaxis normal to and centered on the elliptical base. The major axis ofthe elliptical cone is in the y direction, and the minor axis is in thex direction, and in the plane where the ring 72 comes in contact withthe larger strips 66 the mingr ggis 9f the elliptical cone is equal tothe major axis of the elliptical inside surface 74 on the ring. That is,at the median plane parallel to the x axis there is no bevel on theinside of the ring and there is a maximum bevel at the y median plane.The major axis of the elliptical cone in the plane in contact with thelarger strips is greater than the diameter of the tubular wall 611 sothat the only end surface portion of the ring 72 in contact with thelarger ships 68 comprises a pair of opposed crescent-shaped sectors eachhaving a circular oumide edge and an elliptical inside edge as seen inthe cross section of FIG. 13. The crescent-shaped portions of the ring72 in the plane of the cross section of FIG. 13 are in contact with aplurality of the larger strips 66 near their ends so that liquid passesfrom the wick 711 through the ring 72 into the ends of the largerstrips.

Delivering liquid solely to the ends of the larger strips 68 by thecrescent-shaped portions of the ring 72 in this embodiment is notsufiicient for supplying liquid across the full face of the ventedcapillary vaporizer at maximum heat flux; therefore, in order to supplyliquid to the larger strips at a region intermediate their ends, a pairof similar parallel crossbar structures 77 having their greatest extentin the y direction are provided across the ring. A portion of thecrossbars 77 is illustrated in perspective in FllG. 15 for the purposeof clarifying.

the shape of the crossbar and slope of a pair of outer faces 78. Each ofthe crossbars 77 has a flat face 79 (seen in the cross section of FIG.13) in contact with the larger strips 68 for transferring liquidthereto. The ends 81 of the bars 77 are in contact with, the ring 72 foraccepting liquid therefrom (one of the curved ends 81 is hidden at thefar end of the bar 77 illustrated in FIG.

The two sloping outer faces 78 of the bars are opposite the flat face 79in contact with the larger strips 68. The two faces 78 slope so that thecrossbar is relatively thicker on one side 82 nearer the axis of theheat pipe, and relatively thinner on the opposite side 83 more remotefrom the axis of the heat pipe. The two faces 78 also slope from theirintersection with the ring 72 where they are relatively further from theflat face 79 toward a central intersection of the two faces where theyare relatively nearer the flat face 79. These two directions of slope ofthe faces 78 result in their intersecting along a straight lineinterconnecting a point 84 on the higher side 82, and a second point 86on the lower side 83. The point 86, more remote from the axis of theheat pipe, is at, or nearly at, the flat face 79 of the crossbar, andthe point 84, nearer the axis of the heat pipe, is appreciably moreremote from the flat face 79 than is the other point 86.

The sloping structure of the crossbars 77 is provided for maintaining across section in the bars proportioned to the quantity of liquid thatflows through the cross section as affected by he distance over whichthe liquid must flow through the bar to reach the desired strips 68while also keeping the crosssectional area open for venting the vaporfrom each region proportioned to the vapor flow from that region. Thesame principle is responsible for the bevel 73 on the wick 71 and thebevel 76 on the ring 72. The rationale behind the shape of the crossbarcan be further recognized by noting the dashed lines 87 on FIG. 13 whichrepresent the bounds of the area of surface structure to which liquid isdelivered by one of the crossbars 77. Liquid is delivered to regionsoutside the bounds of the lines 87 by the other crossbar or thecrescentshaped portions of the ring 72. From these bounds it will beseen that the area to which liquid is delivered nearer the center of theheat pipe is larger than the area more remote from the center. It willalso be noted that the distance liquid need flow in the crossbar fromits intersection with the ring to the center is greater on the side 82nearer the center of the heat pipe, and shorter on the side 83 moreremote from the center of the heat pipe.

The sloping faces 78 on the crossbars and the bevels 73 and 76 on thewick and ring, respectively, are provided for enlarging the area throughwhich vapor passes as it leaves the region bounded by the channels 69.The slopes and bevels provide the largest possible cross-sectional areaand shortest path length for the vapor without significant hindering ofliquid flow cross section. It should be recognized that the regionsbounded by the two crossbar-s 77, and between the crossbars and ringform a manifold with and condensing relatively large passages in serialflow connection with the more numerous smaller channels 69 which arefurther in fluid communication with the smallest most numerous channels67 adjacent the heat source surface, thus amuring that all the vaporformed at the regional areas of vaporization on the surface of theporous layer 64 flows into the vapor way within the central passage ofthe wick. In this way, the vented capillary vaporizer illustrated inFIGS. I1 through 16 for a wick-fed heat pipe is analogous to thecompound vented capillary vaporizer hereinabove described andillustrating in FIGS. d and 5 except that instead of feeding liquid tothe vaporizer from a source of bulk liquid, as in FIGS. 4 and 5, theliquid reaches the heat pipe vaporizer by way of a wick 71 and the vaporis delivered to a vapor way instead of being directly returned to mixwith the bulk liquid.

FIGS. 17 through 19 illustrate the fourth type of vented capillaryvaporizer which can be considered to be a capillary pump wherein theoutput vapor pressure can be higher than the input bulk liquid pressure.FIGS. 17 and 18 are cross sections through a single region of heattransfer surface structure and it should be understood that a pluralityof side-by-side structures such as illustrated in FIG. 17 may berepeated indefinitely to cover a large surface area. A single repetitionof the structure such as illustrated in FIG. 17 may, for example,

be provided every one-half inch or more along the surface, and such astructure may have an indefinite length in a direction along that ofFIG. 19 as may be limited only by the flow capacity of the liquid flowconduits, and cooling means provided as hereinafter described. Thestructure illustrated is, of course, only part of the heat transfersystem and elsewhere means are provided for containing and condensing orreleasing the vapor formed.

FIG. 19 is an enlarged detail of the structure adjacent a heat sourcewall 89. Immediately adjacent the wall, and in thermal contacttherewith, are a multiplicity of rectangular strips forming amultiplicity of channels 91 and an equal number of regional areas ofvaporization. Running transversely to the fine strips 91) are largerstrips 92, having channels 93 therebetween, and these larger strips 92are in contact with still larger strips 94, having channelstherebetween. Thus, the structure nearest the heat source provides ahierarchy of strips 94, 92 and 99 of decreasing size and increasingnumber for conducting liquid toward the heat source wall, and ahierarchy of channels 91, 93 and 95 of increasing size and forconducting vapor away from the heat source wall and directing it intothe gaps between adjacent delivery structures for flow into the vaporway common to several structures such as illustrated in FIG. 17 makingup the heat transfer surface structure.

The arrangement provided in FIGS. 17 to 19 dilfers from the relatedstructure hereinabove illustrated in FIGS. 4 and 5 in that liquid isdelivered to the larger strips 94 by a thermal isolation matrix 96 incontact therewith. The thermal isolation matrix is a porous materialhaving a high wicking efficiency e and as low a thermal conductivity aspossible. The structure illustrated in FIGS. 17 and 18 also differs fromthat hereinabove illustrated in FIGS. 4 and 5 in that the vaporgenerated adjacent the heat source surface is collected in a vapor wayexternal to the liquid delivery structures while remaining well isolatedfrom the incoming bulk liquid.

The thermal isolation matrix 96 is in intimate thermal contact with amatrix cooler plate 97 formed of a high thermal conductivity metal.Transverse grooves 98 in the cooler plate 97 transmit liquid fromapproximately triangular conduits 99 to the cooler surface of thethermal isolation matrix 96. One wall of each of the liquid conduits 99is formed by the wall 101 of a conventional heat pipe. The heat pipealso comprises a conventional wick 102 and an open vapor passage 103 forextracting heat from liquid in the conduits 99 and from the matrixcooler plate 97 for delivery to a cooler heat sink (not shown A layer ofthermal insulation surrounds the Heat pipe and liquid conduits and ametal sheath 105 and channel wall 106 prevent liquid in the conduits andvapor surrounding the structure for contacting the insulation. Theliquid channel wall 106 also conducts heat to the heat pipe to help keepthe liquid in the channel cool.

In operation, the'capillary pump-type of vented vaporizer, illustratedin FIGS. 17 and 18, takes bulk liquid that is not in a capillarystructure into the conduits 99 for delivery to the porous thermalisolation matrix 96 by way of the transverse grooves 98.

The bulk liquid entering the conduits 99 is at a relatively lowerpressure than the vapor escaping from the channels 95 nearer the heatsource surface, and the liquid pressure continues to drop as it flowsthrough the conduits 99, grooves 98, and the various capillary matrixstructures. In order to prevent the occurrence of vapor in the liquidflow path, the temperature of the liquid must be suficiently low at anypoint that its vapor pressure at that temperature is less than the sumof the liquid pressure and the bubble pressure at that point. Aspreviously pointed out, the bubble pressure is 08 or o( l/R,+l/R where Rand R are the major radii of curvature of the liquid vapor interface ofthe largest bubble that can form in the pore or channel. when the liquidwets the pore or channel walls, then R and R are approximately half thewidth and half the thickness of the pore or channel, respectively. Thus,for a circular pore of radius R, the bubble pressure is 2o-/R,,. In themicroscopic structures, such as the conduits 99 and grooves 98, R, andR, are generally so large that the bubble pressure is negligible, andthe liquid in them must remain cool enough that the vapor pressure ofthe bulk liquid is less than the liquid pressurev In the very smallpores of the capillary matrix, however, the radii R and R, are also verysmall or, which is equivalent, the effective pore surface to volumeratio 8 is large, so that the bubble premure can be quite high, and thevapor pressure may be considerably greater than the liquid pressurewithout any bubbles forming. Thus, the temperature within such acapillary matrix may be slightly greater than the temperature of thevapor formed at a free surface of the capil lary matrix without formingvapor bubbles in the matrix, while the temperature of the bulk liquidmust be kept enough cooler than the temperature of the vapor formed atthe free surface of the capillary matrix to balance the liquid-vaporpressure difference.

In order to keep the bulk liquid sufficiently cool to prevent itsvaporization, the thermal isolation matrix is selected with as low athermal conductivity as possible for minimizing the quantity of heattransferred from the vapor within the channels 95 through the matrix tothe face in contact with the bulk liquid. Such thermal isolation bymeans of a low thermal conductivity material is sufficient in somecircumstances such as,

for example, as in an apparatus as described and illustrated in theaforementioned copending patent application entitled Heat TransferDevice With isolated Fluid Flow Paths." If the thermal isolation is notsufficient, a heat pipe such as illustrated in FIGS. 17 and 18 isprovided in thermal contact with the bulk liquid for extracting surplusheat therefrom for preventing vaporization. A heat pipe is preferred insuch an application because of the high rate of heat transfer availablewith relatively small temperature difierence; however, it is to beunderstood that other active or passive cooling means than the heat pipecan be employed as desired, such as, for example, a fluid flow coolingtube. or a heat radiating or conducting rod or fin, or a thermoelectriccooling device.

In each of the above-described vented capillary Vaporizers, it ispreferred that the vapor passages in the portion immediately adjacentthe heat transfer surface be spaced apart by less than about 0.1 inch.lfthe passages are spaced apart by appreciably more than about 0.1 inch,the additional expense of preparing the structure is not justified bythe increase obtained in heat flux density The vented capillaryvaporizer structure is more expensive to fabricate than most simple heattransfer structures. Its major advantage is that it is capable ofhandling much higher heat fTufiensities than any but the most expensiveand complex systems, which must use high pressure pumps and under-cooledliquids in order to obtain comparable heat flux densiu'es. This is trueof those devices that exploit in full the highly advantageouscharacteristics of the vented capillary vaporizer.

As shown in equation 60 and 61 hereinabove, the maximum heat fluxdensity H /A is proportional to the total length of vaporizer strip perunit heat source area, which, in turn, is proportional to the number ofpassages or regional areas per unit length n, for passages parallel tothe heat source wall. That is, H /A -n,,. This can also be expressed interms of the nearest neighbor distance d between passages or regionalareas adjacent the heat source surface. HmM/A.-- l/d, or HMI- A ld. Inthis form, the proportionality holds, not only for passages parallel tothe heat source wall, but also for any shape of vented capillaryvaporizer so long as the shape remains unchanged while all dimensionsare scaled proportionately. A particularly interesting example is asquare array of round passages of diameter D perpendicular to the heatsource surface. Here, the maximum heat flux density is approximately thesame as for the passages parallel to the heat flux surface so long asD/d=2/1r since the total effective length of "vaporizer strip" per unitheat source area is the same for both examples. In the terminology usedherein a passage close and parallel to the heat source surface creates asingle regional area of vaporization along it, which for purposes ofcalculation is divided into two vaporizer strips.

In order to evaluate how small the passage spacing a must be made forthe vented capillary vaporizer to be economically competitive, one canbest compare its maximum heat flux density with the maximum obtainablewith the liquid vaporizing situation of pool boiling. Since the heatflux densities obtained are a function of the liquid employed, water isspecified in both examples. Such an example has been set forthhcreinabove for the vented capillary vaporizer as equation 63 wherein itwas pointed out that H,,,,,,/A,,=34.5n, watts/cmf", or substitutingd=l/n,,, the maximum heat flux density is H /A =34.5/ watts/emf.

The maximum heat flux density available for water in pool boiling isabout watts/cmF. Thus, in order to obtain comparable performance34.5/d=l00 watts/cmF, or d=0.345 cm., or 0.136 inch. In order to beeconomically practical, the spacing d should afford some heat fluxadvantage over a simple and inexpensive pool boiling situation. Thus, aspacing at between channels less than about 0.1 inch is preferred.Usually, the spacing between passages or regional areas will be muchsmaller than 0.1 inch since manufacturing costs of the heat transfersurface structure go up less rapidly than the number of channelsproduced until very high values of n are reached, that is, the channelsbecome a very short distance apart. With a heat transfer surfacestructure having channels closer than about 0.1 inch spacing, it isusually less expensive to fabricate a smaller area of higher maximumheat flux density than a lower area of lower maximum heat flux densityhaving the same total heat flux capacity.

Several embodiments of heat transfer surface structures incorporatingprinciples of this invention have been described and illustrated herein.It will be apparent, however, that many modifications can be made inthese structures. Thus, for example, such structures can be employed inheat transfer environments other than in a heat pipe or in a boiler orrefrigeration evaporating tubes as discussed hereinabove. The structureshave been described for vaporization of liquid, however, many of thestructures described and illustrated are also suitable for condensingvapor, such as, for example, at the cooler end of a heat pipe, when thecondensed liquid is removed with a wick or as bulk liquid at sufficienttemperature and pressure to prevent bubble fonnation. The structure isin most cases identical, that is the channel pattern is the same. Thematerial may be different for providing different thermal conductivityand effective pore surface to volume ratio Otherwise, the onlydifierence is that the direction of flow of liquid and vapor is reversedwithin and near the structure. A number of geometrical shapes have beenidentified for structures providing liquid to a heat transfer surface inone area and permitting vapor to escape in another area. Otherstructures performing this function will be apparent to one skilled inthe art.

What I claim is:

l. A heat transfer surface structure comprising:

a heat source surface;

a quantity of vaporizable liquid and its vapor;

a capillary matrix wet by the liquid and having at least a first surfaceportion in thermal contact with the heat source surface, a secondsurface portion in contact with the vaporizable liquid, and a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization spaced sufficientlyclosely to each other and to the first surface portion that the volumeof the capillary matrix through which the liquid must pass between thesecond surface portion and the third surface portion remains liquidfilled;

a multiplicity of vapor passages sufficiently larger than the capillarysize of the capillary matrix to be vapor filled, said vapor passagesbeing in vapor communication between the regional areas of vaporizationof the third surface portion and a region external to the capillarymatrix away from which vapor can flow, and wherein a multiplicity ofsaid vapor passages are embedded substantially into the capillary matrixmaterial. 2. A structure as defined in claim l wherein said regionalareas of vaporization are spaced apart by less than 0.1 inch.

3. A structure as defined in claim 1 wherein the first surface portionis substantially uniformly separated throughout its extent from thesecond surface portion.

4. A heat transfer surface structure comprising: a heat source surface;a quantity of vaporizable liquid and its vapor; a capillary matrix wetby the liquid and having at least a first surface portion in thermalcontact with the heat source surface, a second surface portion incontact with the vaporizable liquid, and a third surface portion fromwhich the principal vaporization of liquid from the capillary matrixtakes place, said third surface portion being arranged as a multiplicityof regional areas of vaporization spaced sumciently closely to eachother and to the first surface portion that the volume of the capillarymatrix through which the liquid must pass between the second surfaceportion and the third surface portion remains liquid filled;

a multiplicity of vapor passages sufficiently larger than the capillarysize of the capillary matrix to be vapor filled, said vapor passagesbeing in vapor communication from the regional areas of vaporization ofthe third surface portion through the capillary matrix to a regionexternal to the capillary matrix away from which vapor can flow.

5. A structure as defined in claim 4 wherein said regional areas ofvaporization are spaced apart by less than 0.1 inch.

6. A structure as defined in claim 4 wherein the capillary matrixcomprises a plurality of bodies of capillary material separated at leastin part by intervening vapor passages.

7. A structure as defined in claim 4 wherein the first surface portionis substantially uniformly separated throughout its extent from thesecond surface portion.

8. A heat transfer surface structure comprising:

a heat source surface;

a quantity of vaporizable liquid and its vapor;

a capillary matrix wet by the liquid and having at least a first surfaceportion is thermal contact with the heat source surface, a secondsurface portion in contact with the vaporizable liquid, and a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization spaced apart lessthan 0.1 inch and sufficiently close to the first surface portion thatthe volume of the capillary matrix through which the liquid must passbetween the second surface portion and the third surface portion remainliquid filled; and 1 a multiplicity of vapor passages suficiently largerthan the capillary size of the capillary matrix to be vapor filled, saidvapor passages being in vapor communication between the regional areasof the third surface portion and a region external to the capillarymatrix away from which vapor can flow.

9. A structure as defined in claim 8 wherein said capillary matrixcomprises:

a first volumetric portion relatively nearer the first surface portionand having a relatively higher thermal conductivity; and

a second volumetric portion relatively further from the first surfaceportion and having a relatively lower thermal conductivity.

10. A heat transfer surface structure comprising:

a heat source surface; I

a quantity of vaporizable liquid and its vapor;

a capillary matrix wet by the liquid and having at least a first surfaceportion in thermal contact with the heat source surface, a secondsurface portion in contact wit the vaporizable liquid, and a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization spaced suficientlyclosely to each other and to the first surface portion that the volumeof the capillary matrix through which the liquid must pass between thesecond surface portion and the third surface portion remains liquidfilled;

a multiplicity of vapor passages sufficiently larger than the capillarysize of the capillary matrix to be vapor filled, said vapor passagesfurther comprising:

a first multiplicity of passages spaced relatively more closely togetherand situated for receiving vapor from the regional areas ofvaporization; and

a second multiplicity of passages spaced relatively less closelytogether and in vapor communication between the first multiplicity ofpassages and a region external to the capillary matrix away from whichthe vapor can flow.

ll. A structure as defined in claim 10 wherein said regional areas ofvaporization are spaced apart by less than 0.1 inch.

12. A structure as defined in claim 10 wherein the capillary matrixfurther comprises:

a first volumetric portion relatively nearer the first surface portionand having a relatively larger effective capillary surface to volumeratio 8, and

a second volumetric portion relatively further from the first surfaceportion and having a relatively smaller efiective capillary surface tovolume ratio 8.

13. A heat transfer surface structure comprising:

a heat source surface;

a quantity of vaporizable liquid and its vapor;

a capillary matrix wet by the liquid and having at least a first surfaceportion in thermal contact with the heat source surface, a secondsurface portion in contact with the vaporizable liquid, nd a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization spaced sufficientlyclosely to each other and to the first surface portion that the volumeof the capillary matrix through which the liquid must pass between thesecond surface portion and the third surface portion remains liquidfilled, and further comprising a first volumetric portion relativelynearer the first surface portion and having a relatively largerefiective capillary surface to volume ratio 8 and a 39111 volumetricportion relatively further from the first surface portion and having arelatively smaller effective capillary surface to volume ratio 8, and

a multiplicity of vapor passages sufficiently larger than the capillarysize of the capillary matrix to be vapor filled, said vapor passagesbeing in vapor communication between the regional areas of vaporizationof the third surface portion and a region external to the capillarymatrix away from which vapor can flow.

14. A structure as defined in claim 13 wherein said capillary matrixfurther comprises:

a first volumetric portion relatively nearer the first surface portionand having a relatively higher thermal conductivity; and

a second volumetric portion relatively further from the first surfaceportion and having a relatively lower thermal conductivity.

15. A heat transfer surface structure comprising:

a heat source surface;

a quantity of vaporizable liquid and its vapor;

a capillary matrix wet by the liquid and having at least a first surfaceportion in thermal contact with the heat source surface, a secondsurface portion in contact with the vaporizable liquid, and a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization

1. A heat transfer surface structure comprising: a heat source surface;a quantity of vaporizable liquid and its vapor; a capillary matrix wetby the liquid and having at least a first surface portion in thermalcontact with the heat source surface, a second surface portion incontact with the vaporizable liquid, and a third surface portion fromwhich the principal vaporization of liquid from the capillary matrixtakes place, said third surface portion being arranged as a multiplicityof regional areas of vaporization spaced sufficiently closely to eachother and to the first surface portion that the volume of the capillarymatrix through which the liquid must pass between the second surfaceportion and the third surface portion remains liquid filled; amultiplicity of vapor passages sufficiently larger than the capillarysize of the capillary matrix to be vapor filled, said vapor passagesbeing in vapor communication between the regional areas of vaporizationof the third surface portion and a region external to the capillarymatrix away from which vapor can flow, and wherein a multiplicity ofsaid vapor passages are embedded substantially into the capillary matrixmaterial.
 2. A structure as defined in claim 1 wherein said regionalareas of vaporization are spaced apart by less than 0.1 inch.
 3. Astructure as defined in claim 1 wherein the first surface portion issubstantially uniformly separated throughout its extent from the secondsurface portion.
 4. A heat transfer surface structure comprising: a heatsource surface; a quantity of vaporizable liquid and its vapor; acapillary matrix wet by the liquid and having at least a first surfaceportion in thermal contact with the heat source surface, a secondsurface portion in contact with the vaporizable liquid, and a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization spaced sufficientlyclosely to each other and to the first surface portion that the volumeof the capillary matrix through which the liquid must pass between thesecond surface portion and the third surface portion remains liquidfilled; a multiplicity of vapor passages sufficiently larger than thecapillary size of the capillary matrix to be vapor filled, said vaporpassages being in vapor communication from the regional areas ofvaporization of the third surface portion through the capillary matrixto a region external to the capillary matrix away from which vapor canflow.
 5. A structure as defined in claim 4 wherein said regional areasof vaporization are spaced apart by less than 0.1 inch.
 6. A structureas defined in claim 4 wherein the capillary matrix comprises a pluralityof bodies of capillary material separated at least in part byintervening vapor passages.
 7. A structure as defined in claim 4 whereinthe first surface portion is substantially uniformly separatedthroughout its extent from the second surface portion.
 8. A heattransfer surface structure comprising: a heat source surface; a quantityof vaporizable liquid and its vapor; a capillary matrix wet by theliquid and having at least a first surface portion in thermal contactwith the heat source surface, a second surface portion in contact withthe vaporizable liquid, and a third surface portion from which theprincipal vaporization of liquid from the capillary matrix takes place,said third surface portion being arranged as a multiplicity of regionalareas of vaporization spaced apart less than 0.1 inch and sufficientlyclose to the first surface portion that the volume of the capillarymatrix through which the liquid must pass between the second surfaceportion and the third surface portion remain liquid filled; and amultiplicity of vapor passages sufficiently larger than the capillarysize of the capillary matrix to be vapor filled, said vapor passagesbeing in vapor communication between the regional areas of the thirdsurface portion and a region external to the capillary matrix away fromwhich vapor can flow.
 9. A structure as defined in claim 8 wherein saidcapillary matrix comprises: a first volumetric portion relatively nearerthe first surface portion and having a relatively higher thermalconductivity; and a second volumetric portion relatively further fromthe first surface portion and having a relatively lower thermalconductivity.
 10. A heat transfer surface structure comprising: a heatsource surface; a quantity of vaporizable liquid and its vapor; acapillary matrix wet by the liquid and having at least a first surfaceportion in thermal contact with the heat source surface, a secondsurface portion in contact with the vaporizable liquid, and a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization spaced sufficientlyclosely to each other and to the first surface portion that the volumeof the capillary matrix through which the liquid must pass between thesecond surface portion and the third surface portion remains liquidfilled; a multiplicity of vapor passages sufficiently larger than thecapillary size of the capillary matrix to be vapor filled, said vaporpassages further comprising: a first multiplicity of passages spacedrelatively more closely together and situated for receiving vapor fromthe regional areas of vaporization; and a second multiplicity ofpassages spaced relatively less closely together and in vaporcommunication between the first multiplicity of passages and a regionexternal to the capillary matrix away from which the vapor can flow. 11.A structure as defined in claim 10 wherein said regional areas ofvaporization are spaced apart by less than 0.1 inch.
 12. A structure asdefined in claim 10 wherein the capillary matrix further comprises: afirst volumetric portion relatively nearer the first surface portion andhaving a relatively larger effective capillary surface to volume ratiodelta , and a second volumetric portion relatively further from thefirst surface portion and having a relatively smaller effectivecapillary surface to volume ratio delta .
 13. A heat transfer surfacestructure comprising: a heat source surface; a quantity of vaporizableliquid and its vapor; a capillary matrix wet by the liquid and having atleast a first surface portion in thermal contact with the heat sourcesurface, a second surface portion in contact with the vaporizableliquid, and a third surface portion from which the principalvaporization of liquid from the capillary matrix takes place, said thirdsurface portion being arranged as a multiplicity of regional areas ofvaporization spaced sufficiently closely to each other and to the firstsurface portion that the volume of the capillary matrix through whichthe liquid must pass between the second surface portion and the thirdsurface portion remains liquid filled, and further comprising a firstvolumetric portion relatively nearer the first surface portion andhaving a relatively larger effective capillary surface to volume ratiodelta and a second volumetric portion relatively further from the firstsurface portion and having a relatively smaller effective capillarysurface to volume ratio delta , and a multiplicity of vapor passagessufficiently larger than the capillary size of the capillary matrix tobe vapor filled, said vapor passages being in vapor communicationbetween the regional areas of vaporization of the third surface portionand a region external to the capillary matrix away from which vapor canflow.
 14. A structure as defined in claim 13 wherein said capillarymatrix further comprises: a first volumetric portion relatively nearerthe first surface portion and having a relativelY higher thermalconductivity; and a second volumetric portion relatively further fromthe first surface portion and having a relatively lower thermalconductivity.
 15. A heat transfer surface structure comprising: a heatsource surface; a quantity of vaporizable liquid and its vapor; acapillary matrix wet by the liquid and having at least a first surfaceportion in thermal contact with the heat source surface, a secondsurface portion in contact with the vaporizable liquid, and a thirdsurface portion from which the principal vaporization of liquid from thecapillary matrix takes place, said third surface portion being arrangedas a multiplicity of regional areas of vaporization spaced sufficientlyclosely to each other and to the first surface portion that the volumeof the capillary matrix through which the liquid must pass between thesecond surface portion and the third surface portion remains liquidfilled, and further comprising a first volumetric portion relativelynearer the first surface portion having a relatively higher thermalconductivity and a second volumetric portion relatively further from thefirst surface portion and having a relatively lower thermalconductivity; and a multiplicity of vapor passages sufficiently largerthan the capillary size of the capillary matrix to be vapor filled, saidvapor passages being in vapor communication between the regional areasof vaporization of the third surface portion and a region external tothe capillary matrix away from which vapor can flow.
 16. A structure asdefined in claim 15 wherein the first surface portion is substantiallyuniformly separated throughout its extent from the second surfaceportion.
 17. A heat transfer surface structure comprising: a heat sourcesurface; a quantity of vaporizable liquid and its vapor; a capillarymatrix wet by the liquid and having at least a first surface portion inthermal contact with the heat source surface, a second surface portionin contact with the vaporizable liquid, and a third surface portion fromwhich the principal vaporization of liquid from the capillary matrixtakes place, said third surface portion being arranged as a multiplicityof regional areas of vaporization spaced sufficiently closely to eachother and to the first surface portion that the volume of the capillarymatrix through which the liquid must pass between the second surfaceportion and the third surface portion remains liquid filled; amultiplicity of vapor passages sufficiently larger than the capillarysize of the capillary matrix to be vapor filled, said vapor passagesbeing in vapor communication between the regional areas of vaporizationof the third surface portion and a region external to the capillarymatrix away from which vapor can flow; means for thermally insulatingthe fluid adjacent the second surface portion of the capillary matrixfrom portions of the external ambient environment having a highertemperature than said fluid.
 18. A heat transfer surface structurecomprising: a heat source surface; a quantity of vaporizable liquid andits vapor; a capillary matrix wet by the liquid and having at least afirst surface portion in thermal contact with the heat source surface, asecond surface portion in contact with the vaporizable liquid, and athird surface portion from which the principal vaporization of liquidfrom the capillary matrix takes place, said third surface portion beingarranged as a multiplicity of regional areas of vaporization spacedsufficiently closely to each other and to the first surface portion thatthe volume of the capillary matrix through which the liquid must passbetween the second surface portion and the third surface portion remainsliquid filled; a multiplicity of vapor passages sufficiently larger thanthe capillary size of the capillary matrix to be vapor filled, saidvapor passages being in vapor communication between the regional areasof vaporization of the third surface portion anD a region external tothe capillary matrix away from which vapor can flow; means for removingheat from the fluid adjacent the second surface portion of the capillarymatrix, said means being capable of removing said heat even when,without said means, the region surrounding said fluid would have atleast as high a temperature as the fluid.
 19. A heat transfer surfacestructure comprising: a heat source surface; a quantity of vaporizableliquid and its vapor; a capillary matrix wet by the liquid and having atleast a first surface portion in thermal contact with the heat sourcesurface, a second surface portion in contact with the vaporizableliquid, and a third surface portion from which the principalvaporization of liquid from the capillary matrix takes place, said thirdsurface portion being arranged as a multiplicity of regional areas ofvaporization spaced sufficiently closely to each other and to the firstsurface portion that the volume of the capillary matrix through whichthe liquid must pass between the second surface portion and the thirdsurface portion remains liquid filled; a multiplicity of vapor passagessufficiently larger than the capillary size of the capillary matrix tobe vapor filled, said vapor passages being in vapor communicationbetween the regional areas of vaporization of the third surface portionand a region external to the capillary matrix away from which vapor canflow; active means for removing heat from the fluid adjacent the secondsurface portion of the capillary matrix.
 20. A structure as defined inclaim 19 wherein the active means for removing heat comprises a heatpipe.
 21. A structure as defined in claim 19 wherein the active meansfor removing heat comprises a thermoelectric cooling device.
 22. A heattransfer surface structure comprising: a heat sink surface; a quantityof fluid consisting of a vaporizable liquid and its vapor; a capillarymatrix wet by the liquid and having at least a first surface portion inthermal contact with the heat sink surface, a second surface portionthrough which the fluid is withdrawn from the capillary matrix, and athird surface portion at which the principal condensation of vapor takesplace; said third surface portion being arranged as a multiplicity ofregional areas of condensation; a multiplicity of vapor passagessufficiently larger than the capillary size of the capillary matrix tobe vapor filled; said vapor passages being in vapor communicationbetween the regional areas of condensation of the third surface portionand a region external to the capillary matrix from which the vaporpassages receive vapor.
 23. A structure as defined in claim 22 wherein amultiplicity of said vapor passages are embedded substantially into thecapillary matrix material.
 24. A structure as defined in claim 22wherein a multiplicity of said vapor passages are in vapor communicationfrom the regional areas of condensation through the capillary matrix tothe region external to the capillary matrix from which the vaporpassages receive vapor.
 25. A structure as defined in claim 22 whereinsaid regional areas of condensation are spaced apart by less than 0.1inch.
 26. A structure as defined in claim 22 wherein said vapor passagesfurther comprise: a first multiplicity of passages spaced relativelymore closely together and so as to deliver vapor to the regional areasof condensation; and a second multiplicity of passages spaced relativelyless closely together and in vapor communication between the firstmultiplicity of passages and the region external to the capillary matrixfrom which the vapor passages receive vapor.
 27. A structure as definedin claim 22 further comprising means for maintaining the pressure of theliquid in the capillaries of the capillary matrix lower than thepressure of the vapor adjacent the third surface portion of thecapillary matrix.
 28. A structure as defined in claim 22 wherein saidcapillary matrix comprises: a first volumetric portion relatively nearerthe first surface portion and having a relatively higher thermalconductivity; and a second volumetric portion relatively further fromthe first surface portion and having a relatively lower thermalconductivity.